In recent years, air bag systems have found wider use in vehicles such as automobiles. Such air bag systems include an air bag, which functions as an air bag for use with a shock absorbing device. The air bag is folded and housed inside a storage container. When a vehicle collision is detected by a sensor, the air bag is instantaneously inflated by a gas supplied from a gas generator, whereby the airbag functions as a cushion for absorbing shocks applied to the driver or passenger.
The air bag is covered with a resin-made interior member, such as a steering wheel cover or an instrument panel. When the air bag is inflated, a door panel of the resin-made interior member for deploying the air bag is forcibly opened and exposed outside of the storage container.
In order to open the door panel reliably, the door panel includes a fragile region, having a certain processed pattern such as grooves, holes, or the like formed therein.
It is known to form the fragile region by means of a processing method employing a heated blade or a laser beam. According to one proposed processing method that employs a laser beam, the position of the surface of the interior member is measured by a distance sensor, located at a given position, and grooves having a desired depth are formed by the laser beam while an output power of the laser beam is controlled based on the measured position. For details, reference should be made to Japanese Laid-Open Patent Publication No. 2001-300748 and Japanese Laid-Open Patent Publication No. 8-282420. In the processing method, a multi-axis robot is used as a moving mechanism.
Since the output power of the laser beam is controlled, the controller that is used is complex in structure, and the manufacturing costs therefor are high. For controlling the output power of the laser beam, it is necessary to directly operate an oscillating unit in a laser oscillator, which results in the apparatus being complex, and the control procedures for the apparatus are complicated. The grooves of the fragile region, which are formed in the thin instrument panel, need to be minute in size. However, it is practically difficult to only slightly change the output power of the laser beam, for appropriately shaping such minute grooves.
If the fragile region is constructed as an array of successive minute holes, in terms of quality of appearance, it is preferable to make the diameter of the minute holes on the surface sufficiently small and uniform, so as to be invisible.
The fragile region may be shaped as a single swing door panel, or as double swing door panels.
If the fragile region is shaped as double swing door panels, then it has been proposed to divide linear parts of the fragile region into outer edge corners, outer edge straight portions and a central boundary, as well as to change the rupture strengths thereof, for allowing the door panels to be smoothly released while preventing fragments from scattering around (see, for example, Japanese Laid-Open Patent Publication No. 2002-144999). According to this proposal, the rupture strength of the central boundary is equal to or greater than the rupture strength of the outer edges, so that the outer edges will be ruptured before the central boundary.
With the fragile region proposed in Japanese Laid-Open Patent Publication No. 2002-144999, different rupture strengths are achieved by changing the depth of grooves or the pitch of recesses, depending on a processing interval. However, it is difficult to obtain an appropriate rupture strength depending on the set location, simply by changing the depth of the grooves or the pitch of the recesses.
When a laser beam is applied to an interior member to form a fragile region therein, the interior member is held and moved by a robot, for example, at a constant speed and along a predetermined path with respect to the position where the laser beam is applied. By continuously applying the laser beam at an appropriate output level, the fragile region is produced in the form of a groove having a predetermined depth. By applying a pulsed laser beam, the fragile region is formed as perforations, in the shape of minute holes spaced at suitable intervals.
Immediately after the moving mechanism, such as a robot or the like, has started moving from a stopped state, the speed or the direction at which the moving mechanism moves may be somewhat unstable. If the laser beam is applied immediately after the moving mechanism has started moving, then the depth of the grooves or the pitch of the minute holes within the formed fragile region tends to become irregular, or the direction in which the fragile region extends is liable to become shifted from a predetermined direction.